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Predictions should come with a health warning

Fergus Walsh | 11:45 UK time, Friday, 3 July 2009

A lot of people got quite a shock yesterday when Andy Burnham, the health secretary, predicted that there could be 100,000 people a day infected with H1N1 swine flu by the end of August.

liam donaldsonAnd then on Newsnight, the chief medical officer for England, Sir Liam Donaldson was asked whether that meant that 40 people a day might be dying from swine flu:

"Well we can't be sure about the death rate, nor can we be absolutely sure about the number of cases that will be occurring by the end of August. But these are scientific projections, the numbers could be lower than that, they may be even higher.'"

So there you have it. Predictions, projections, but not facts. The only facts we have are that there have been around 7,500 lab-confirmed cases of swine flu (thousands more unconfirmed) and three deaths. That makes a death rate of one per 2,500 cases or 40 per 100,000.

Could the 40 deaths a day figure be accurate? The US experience may help us. They are about one month ahead with swine flu and the CDC reckons (here comes another estimate) that at least one million Americans have had swine flu. They made that prediction when there had been 127 deaths in the US. So, using the American figures, that would give us a UK death rate of one per 7,800 cases or 12 deaths a day come the end of August.

Which goes to show that predicting death rates, and even case numbers, is an inexact science, and such predictions should come with a health warning.

One figure which people should not be surprised by is the number of cases. One hundred thousand a day seems perfectly possible at the peak of a national epidemic. The real surprise will be if it comes in August, even before children return to school.

government's pandemic plan from November 2007This large "attack rate" of a pandemic virus is to be expected when so many are exposed to a new virus. The government's pandemic plan from November 2007 predicted (oops, there we go again) that 25% of the population might get infected. That's 15 million people.

The infection will probably come in a few waves spread over about two years. A daily rate of 100,000 infections would take you to 10 million people in 100 days. I'm not predicting that, just doing the maths. I certainly think it's too early to predict eventual death rates with any certainty.

What no-one has quite worked out yet is the number of people who are asymptomatic - that is, they have the virus in their system but show no (or few) signs of ill-health. This rate must be pretty high and it helps explain why the disease is spreading so quickly, because symptomless people are less careful when mixing with others. It's another reason why basic hygiene is crucial.

Let's stick then with what we do know. The majority of people so far infected have had a mild illness and recovered completely without the help of antiviral medicines. Some with swine flu will simply feel a bit grotty for a few days. For very many, uncomplicated swine flu will still be an unpleasant experience - laid low in bed with a sudden fever and cough.

Add to that a selection of the following: headache, aching limbs, lethargy, joint pain, diarrhoea, stomach upset, chills - I could go on. That still classifies as a mild illness, because after a few days in bed feeling awful, you get over it. Those with underlying health conditions and pregnant women are certainly at greater risk but they too should not be unduly alarmed, but vigilant for the symptoms.

And once again, as per medical advice, please turn down any invitations to swine flu parties (do they really exist?). It does not pay to expose yourself to this virus about which much is still unknown, and you could spread it to others who are less robust. Remember that about a quarter of the deaths so far in the US have been among people who had no health problems. I would prefer not to risk my health, but hold tight for the vaccine.

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